The present invention relates to shielding materials and, more particularly, to a new, multi-layered particle of metal coated ferrites coated with a conductive polymer, and networked into a conductive polymer matrix to produce single-layered shielding coatings, films and sheets having a tailored application in sophisticated electronics and military hardware.
Sophisticated electronics, particularly those employing high frequencies and military hardware using space-age composites, require electromagnetic radiation hardening. Survivability countermeasures use shielding against nuclear, laser, EMI and radar/microwave radiation. Several hardening methods exist. The choice of which techniques to use depends on the frequency range and the type of the electromagnetic threat. Surface metallization, use of conductive composites, and metallic enclosures, are but a few of the shielding techniques employed. Each method has its own shortcomings, and the materials used are often expensive.
It is not uncommon to observe shielding failures in composites resulting from radiation transparent spots, phase separations, and cracking. Coating imperfections, corrosion in metallized components, scratches, and feed-through holes in metallic enclosures, represent additional shielding failure problems.
No single material satisfies shielding requirements for a wide range of frequencies; therefore, several materials and methods are employed in order to achieve a desired absorption. Materials whose composite layers have different functions are currently used. These materials are expensive and inefficient, and their composite layers add weight to the assembly. The weight drawback is particularly anathema to military, aeronautical, and space objectives.
Blends of different materials have been made to satisfy the shielding requirements for use in a wide frequency range. However, each material in the blend is diluted by virtue of its inclusion, thus weakening the original properties.
Manufacturers and suppliers have developed a number of electrically conductive compounds that incorporate conductive fillers. They have also provided techniques for coating molded plastic surfaces with metals including copper, silver, nickel and their alloys. The incorporation of conductive particles in an insulating matrix requires that the amount of particles be higher than the percolation threshold concentration. The same rule must be applied when adding a second conductive component with different EM absorption, dissipation, or reflection characteristics.
The complexities of the shielding problem are further exacerbated by continually changing specifications that demand ever higher levels of shielding protection. ASTM Committee D9 has developed and issued a new standard for testing shielding effectiveness (SE): ASTM ES 7-83. Also a new edition of IEC 60601-1-2, Collateral Standards of EMC Requirements and Tests of Medical Electrical Equipment, is currently being debated.
The use of polymeric materials having high conductivity has also been explored in radiation hardening applications. The polymeric materials are often combined with other shielding materials. The conductive polymeric materials can be used over a wide frequency spectrum, owing to their interesting electromagnetic characteristics. Conductive polymeric materials include the class of doped conjugated polymers such as polypyrroles, polyanilines, polythiophenes, poly(3,4-ethylenedioxythiophene), polyphenylenes and polypheneylene vinylenes and derivatives thereof.
Their use in EMI applications is strongly dependent on their conductivity and permeability. High dielectric constants, derived from their dynamic conductivity, make the conductive polymers ideal for microwave hardening and radar absorption applications.
Conductive polymers are typically synthesized using well known procedures. They usually have high molecular weight. They are electron-rich due to their conjugated backbone and their ability to give or accept electrons. Although these polymers have metallic conductivity, conduction is driven by a different mechanism from that governing metals, or inorganic semiconductors. In metals, electrons move by hopping, while in most known conductive polymers, the charge carriers are polarons and bipolarons. These electronic states correspond to energy levels within the band gap, thus making them intrinsically conductive. Therefore, the frequency dependence of their conductivity and their dielectric constant is different from those of metals.
U.S. Pat. Nos. 5,938,979 and 6,080,337, both for ELECTROMAGNETIC SHIELDING, issued Aug. 17, 1999 and Jun. 27, 2000, respectively, to Kambe et al., teach electromagnetic shielding material formed from a shielding composition made with magnetic particles and a binder. The magnetic particles have an average diameter less than about 1000 nm and are substantially crystalline. The magnetic particles can be formed from Fe2O3, Fe2O4, Fe3C, or Fe7C3. The shielding composition can be formed into a layer or into composite particles. The binder can be a metal or an electrically conducting polymer. A conducting layer can be placed adjacent to the shielding composition.
The present invention reflects the use of new shielding materials using a novel, three-layered particle. The particle is a conductive polymer disposed over a metal-coated, ferromagnetic particle to form the three-layered, conductive, ferromagnetic particle. The new, three-layered particle is blended into a polymer matrix and processed in a magnetic field to form single-layered coatings and freestanding films and sheets. The blends are magnetically processed, such that a conductive network of particles is obtained within the polymer matrix, rather than having discrete particles disposed within a medium. The preferred metal layer of the particle comprises nickel, because of its ferromagnetic and conductive properties. The metal layer also comprises other ferromagnetic and non-ferromagnetic metals such as silver, manganese, aluminum, magnesium and zinc. A typical conductive polymer coating and matrix material can comprise polypyrroles, polythiophenes, polyanilines and other similar materials from the class of intrinsically conductive polymers.
Radar uses electromagnetic waves that bounce off of a particular target, and are collected by a receiver that analyzes the reflected signal. The range, direction and speed of the object is then determined. Reflections occur whenever there is a sharp impedance difference between the medium (usually air) and the object. Metals tend to re-radiate or reflect the incoming signal. Conductive polymers as radar absorbers in antennas, Salisbury screens, camouflage, and other types of shielding are of interest to the military. Conductive polymer camouflage reflects back differently form the object it covers. It absorbs microwave radiation, because it has more continuously variable impedance. A conductive polymer textile used for camouflage has no sharp edges, or wings, and tends to appear indistinguishable form its surroundings. Microwave (100 MHz-12 GHz) properties of conductive polymer fabrics have been studied.
Stealth aircraft could benefit from the inventive materials. The metallic aircraft surface is a reflector with respect to EM waves. That is why for twenty years much work has been devoted in the U.S., Europe and former USSR to the concept of radar absorbing materials (RAM) associated with an optimized shape of the aircraft.
However, all of the above work, apart from the camouflaging textiles and shielded cables, use synthesized polymers with no variation in characteristics or parameters for their intended applications.
The new particles and their novel films and sheets provide the following novelties and advantages, heretofore unknown in the art:
1. High permeability ferrites coated with a ferromagnetic metal, and an inherently conductive polymer with high conductivity and interesting dielectric properties is combined into one single particle. For purposes of this disclosure, the term xe2x80x9cferritesxe2x80x9d is meant to include magnetite.
2. Conductivity and frequency response of the fabricated materials can be tailored for specific products.
3. The conductive polymer layer of the novel particles is used both as a shielding material and as a plasticizer/binder. The conductive polymer comprises any polymer from the class of intrinsically conductive polymers.
4. Lightweight shields can be formed due to the low percolation threshold.
5. The shields fabricated from the novel films and sheets can be repaired easily in the field, if they are physically damaged.
6. The synthesized materials are inexpensive, and are easily fabricated using straightforward, state of the art, synthesis techniques.
In accordance with the present invention, there are provided new shielding materials using a novel particle. The particle is a conductive polymer disposed over a metal-coated, ferromagnetic particle to form a three-layered, conductive, ferromagnetic particle. The new three-layered particle is blended into a polymer matrix and processed in a magnetic field or without a magnetic field to form single-layered coatings and freestanding films and sheets. The blends are magnetically processed, such that a conductive network of particles in the matrix is obtained, rather than discrete particles disposed within a medium. The ferrite particle is coated with the metal layer. Then the particles are blended in a conductive polythiophene derivative which is in solution or dispersion form. The can also be coated electrochemically with an intrinsically conductive polymer such as polypyrrole or polythiophene. The polymer is rendered conductive by virtue of the doping process, the dopant being molecular such as toluene sulfonate or polymeric such as polystyrenesulfonate. Nickel is the preferred metal for the metal coating of the particles, because of its ferromagnetic and conductive properties. However, other metals such as silver, manganese, aluminum, magnesium and zinc work as well. A typical conductive polymer coating and matrix can comprise polypyrroles, polythiophenes and similar intrinsically conductive polymers.
It is an object of this invention to provide improved shielding materials having wide applicability in space, aeronautics, and military applications.
It is another object of the invention to provide improved shielding materials having low weight and low cost.
It is a further object of this invention to provide improved shielding materials having applicability over a wide frequency range, and which can be tailored to a specific application.